Few events in Chicago's musical history have attracted as much attention, or galvanized as much controversy, as the creation of the cooperative Association for the Advancement of Creative Musicians. Through the European travels of the Art Ensemble of Chicago and the individual efforts of such artists as Anthony Braxton and Muhal Richard Abrams, the AACM came to emblematize a unique midwestern take on the issues of freedom and space that faced jazz in the 1960s; to many listeners around the world it represented the "avant-garde" at large. As a means of celebrating the 30th anniversary of the AACM, this year's Chicago Jazz Festival has quite literally resurrected the organization's prehistory. The musical and sociological impetus to form the AACM came from Abrams's "Experimental Band," an ongoing jazz-orchestra workshop that the pianist conducted on Chicago's south side starting in 1962. Abrams has since led an almost bewildering array of groups, creating an oeuvre at once daring and accessible, but he saw the Experimental Band as something else--a sort of investigative work in progress that would spur growth and new ideas, as opposed to a concertizing ensemble. (The Experimental Band did not play in public.) This loose-limbed laboratory attracted a grand array of musical freethinkers--some of whom will return for this event--and left its mark on the dozens of others who would become charter members of the AACM. Among those who will breathe life into Abrams's visionary concept: saxists Roscoe Mitchell, Joseph Jarman, Anthony Braxton, and Henry Threadgill; trumpeter Leo Smith and trombonist George Lewis; violinist Leroy Jenkins; and keyboardist/vocalist Amina Claudine Myers. This concert represents one of the most ambitious undertakings in years for the Jazz Festival and for the Jazz Institute of Chicago, which programs the festival; it gathers artists from both coasts and from Europe, and includes the commissioning of several new works composed for the occasion by Abrams (and thus receiving their world premieres at this concert). For those who appreciate adventurous yet rigorously organized music, the Abrams band promises to be the jewel on this, the crowning night of the festival, which also features John Scofield's quintet (with the inventive tenor great and native Chicagoan Eddie Harris) and a 75th-birthday tribute to Clark Terry, the ageless Saint Louis-born trumpeter who answers the musical question: who is the only man ever to star in the bands of Count Basie, Duke Ellington, and Doc Severinsen (back when the Tonight show emanated from New York)? Sunday, 6:50 PM, Petrillo Music Shell, Grant Park, Columbus and Jackson; 744-3315.